Frequency modulation



Nov. 27, 1945. H TUNlCK FREQUENCY MODULATION Original Filed Marci: 27, 1942 2 Sheets-Sheet 1 INVENTOR ziwway EN/C'K BY 7% ATTORNEY Ella,

' Nov; 27, 1945. H. IUNICK FREQUENCY MODULATION Original Filed M rch 27, 1942 2 Sheets-Sheet 2 INVENTOR MmaP/m/v: we Arm-0 MOW M zgm y 0 mmvm wi Patented Nov. 27, 1945 maousncr MODULATION Harry Tunick, Rye, N.,Y., asslgnor to Radio Cor- Wration of America, a corporation of Delaware I ommn application March 27, 1942. serial No. 436,428. Divided and this application February 9, 1944, Serial No. 521,632

, 13 Claims.

This is a division of my copending application Serial No. 436,423, filed March 27, 1942.

My present invention relates to signaling by means of frequency modulated waves. An object of my present invention is to provide improved circuits especially adapted for telegraphic communication by frequency shifted waves. objects, advantages and features of my present invention will be self-evident as the more detailed description thereof proceeds.

Briefly, in accordance with one form of 'my present invention, a quadrature excited reactance tube connected to an oscillation generator is provided with keying circuits whereby the conductivity of the reactance tube is abruptly changed, thereby abruptly shifting the frequency of the oscillator. Such an arrangement is particularly useful for telegraphic signaling by means of frequency shifted waves.

In the accompanying drawings:

Figures 1, 1a and 1B are circuit diagrams of oscillators which may be keyed from one frequency to another; and

Figure'.2 is a wiring diagram of a radio frequency, frequency modulatedtransmitter which oi! keying or for telephony.

A form of tone generator which may be used in telegraphic tone-transmitting systems is illustrated in Figure 1. Vacuum tube y i. is connected as illustrated to produce oscillations of audio frequency, the plate coil PCL being of the powdered iron core type. The reactance tube RT is provided with a phase shifting network consisting of resistor R8 .and an iron cored phase shifting coil LI. Condenser BC is a by-pass condenser and of low reactance for oscillations generated by oscillator tube VT. Resistor R8 is of low resistance relative to the value of reactance of coil LI.

With the key KR open, it will be observed that the oscillator RT of Figure 1 operates at a irequency determinedby the tuning of the iron cored inductor PCL shunted-by variable condenser C in the plate circuit as modified by'the reactance tube RT. With the switch KR closed, RT may becut off or caused to draw a lesser current, as a result of which vacuum tube VT oscillates at a different frequency. In short, by manipulation of key. KR, the conductivity of reactance tube RT is abruptly changed, thereby Other VT operated as a high frequency oscillator, in which case the output of the oscillator may be radiated with or without frequency multiplication and amplification. When this is done, two discreet frequencies are successively radiated by operation of key m Figure 1A is an arrangement similar in most respects to Figure 1. In Figure 1A, however, the resistance R8 is connected between the plate and control grid of reactance tube RT. Also, the resistance of R8 is high with respect to the reactance of coil LI connected between the control grid and cathode of tube RT. In other words,

'25 may be used for keyed tone telegraphy, on andabruptly shifting the frequency of oscillator VT from one value to another.

If desired, as shown in Figure 1B coil LI of Figure 1 may be replaced by a condenser CI and tube,

the value of the resistance RS is so chosen with respect to the inductive reactance of coil LI that the phase anfle of the entire circuit from point P to ground G is practically that of RS. As'a result, the voltage at point Q is substantially degrees out of phase or in quadrature with the phase of the voltage across P-G.

Separate cathode return biasing resistors BR! and BR! are provided in Figure 1A as distinguished from the common biasing resistor BR of Figure 1. In Figures 1 and 1A condensers HC are by-passing condensers and OR is an additional condenser-grid leak circuit for supplying additional bias on the control grid ofthe oscillator tube YT.

The transmitting system of Figure 2 consists of the push-pull oscillator tubes V'I'l and VT! provided with a tuned output circuit TO whose coil C may be ofthe iron core or powdered iron core type. To produce oscillations, feedback-condensers FCI and FC2 a provided. Vacuum tubes VT! and VT2 may I of course. be of the screen grid or pentode ty The reactance tubes RTI, RT2 are provided L2 and LI which may be provided with adjustable iron or powdered iron cores! The reactance tubes RT! and R'Il are provided with phase shifting condensers C3 and C4.

By virtue of their connection, as described, reactance tubes-RTI and RT! produce in shunt to the tuned circuit TO a reactance effect of one sign when they are permitted to become conductlve, whereas reactance tubes RT! and RT! produce in shunt to the output circuit TO a reactance of. the opposite sign, thereby changing the frequency of oscillation up or down, depending upon which of the palrs oi' tubes are made conductive. The amount of reactance so shunted into circuit and, hence, the extent of thefreing of by-pass con-- quency change varies with the reactance tubes.

Resistors 2, 4, 6, 8 are cathode return biasing resistors which may, if desired, be individually shunted with by-pass condensers. Resistors Ill and I2 are of low value relative to the inductive reactance of inductors L2 and L3. Similarly, resistors l4, it are of low value relative to the capacitive reactance of condensers Cland C4,

Resistors l8 and 20 are cathode return biasing resistors for the grids of oscillator tubes VTI and VT2 which may be used in addition, if desired, to the negative bias inserted through lead 22. Also, if desired, the negative bias impressed through lead 22 may be made sufficiently high so that in the absence of signal at transformer TV, tubes VTI and VT2 are cut oil. When signal is impressed through M, however, a part of the same, amplified by amplifier VA, is picked up by coil 24, rectified by rectifier 26 and fed across resistor 28 in lead 22 in such a direction as to unblock tubes VTI and VT2.

It is preferred, however, to omit coil 24, rectiiler 26, resistor 28 and permit tubes VTI and VT2 to oscillate at full power by proper choice of bias through lead 22, even in the absence of signal at microphone M. This, it will be recognized, is the usual way of operating an F. M. transmitter.

Normally, the reactance .tubes RTI, RT2, RT3 and RT! of Figure 2 are biased to cut-off by their screen grids 32, 34,- 36 and "which are biased to cut-off by battery or other voltage source BB v supplying voltage through leads LLI and IL2.

Manipulation of the key KS to the left contact CS I causes tubes RT3 and RTl to become abruptly conductive, whereas moving the key KS tothe other contacts CS2 causes tubes RTi and RT2 to become suddenly conductive while simultaneously causing the abrupt blocking of the previously unblocked tubes RT3 and RT. Short-circuiting of the biasing battery or source BB during the keying operation is prevented by the action of resistors R1, R8 which act in a way similar to the resistor R9 of Figure 1.

By way of further explanation, it will be found that reactance tubes RTi and RT2 act as variable inductances. As the conductivity of these tubes is increased, th eifective inductance in shunt to the circuit T0 is increased, thereby.

lessening the total inductance of circuit T0 causing an increase in frequency. Reactance tubes RT3 and RT! act as capacitors eifectively in shunt to the tuned circuit TO. Hence, when the conductivity of tubes RT3 and RT is increased, the eflective capacity of circuit T0 is increased, thereby decreasing the frequency of the waves fed to the output circuit.

For voice modulation of the transmitting system of Figure 2, switch K8 is left open in its mid position as shown and switch Sll is closed, enabling voice currents generated by pick-up microphone M and amplified by the voice amplifier VA to be impressed through transformer TV in phase opposition upon the. screen grids of the pairs of tubes RTI, RT2 and RTS, RT. If de- I sired, an additional biasing source BSll may be provided and connected, as shown, to partially unblock the reactance tubes when the transmitter is used for transmission of speech or other undulatory waves supplied to former TV.

Tone oscillators, as explained in my United States Patent No. 2,282,102, may be used for the conductivity'of in which case keyed tones are fed into the primary of transformer TV and switch Slfl is clor .d, but switch K8 is left in its open mid-position. The output of the transmitter of Figure 2 may be fed, regardless 01' the type of modulation used (speech or keyed tones), to frequency multipliers and amplifiers before radiation over a transmitting antenna.

- Having thus described my invention, what I claim is:

1. In combination, an oscillation generator, a reactance tube having an anode, a cathode and a control electrode connected thereto for changing the frequency of oscillation of said oscillation generator, a reactance circuit-for supplying quadrature voltages to the control electrode of said reactance tube from said oscillation generator, and a keying circuit for abruptly changing the conductivity of said reactance tube from one value when the key is open to another value when the key is closed.

2. In combination, an oscillation generator, a pair of reactance tubes each having an anode, a cathode and a control electrode, means for feeding quadrature voltages of opposite sign to the control grids of said reactance tubes, and a keying circuit for rendering one of said reactance tubes conductive while rendering the other nonconductive and vice versa whereby waves generated by said generator are abruptly changed in frequency from one value to another.

3. In combination, an oscillation generator, 2. reactance tube having an anode, a cathode and a control electrode connected thereto for changing the frequency of oscillation of said oscillation generator, 8. reactance circuit for supplying quadrature voltages to the control electrode of said reactance tube from said oscillation generator, a source ofuni-directional potential having one terminal connected toathe cathode of said reactance tube, and a key for connecting, at will, a control electrode of said reactance tube to either of two points of diiferent potential on said source.

4. In combination, a resonant electrical circuit, a reactance tube having an anode, a cathode and a control electrode connected thereto for changing the effective resonant frequency of said circuit, a reactance circuit for supplying quadra-' v ture voltages to the control electrode of said reconductive and vice versa.

the 'trans- 6. In combination, an oscillation generator, I. reactance tube having an anode, a cathode, a control electrode, and an auxiliary grid, means for subjecting the electrodes of said tube to suitable operating potentials, said tube being connected to said generator for changing the frequency or oscillation of said oscillation generator, I. reactance circuit connected to the generator and to said control electrode for nippiyingquadrature volt- Ises tothe control electrode of said reactance tube from said oscillation generator, a source of transmission .over the transmitter of l'lsurei 2, 78 uni-directional potential having'one terminal c nected to the cathode of said reactance'tube, a resistance connected to another point on said source and to said auxiliary grid, and a key con- 7. In combination, an oscillation generator, areactance tube having an anode, a cathode, a control electrode and a screen grid, circuits for subjecting the electrodes of said reactance tube to suitable operating potentials, said tube being connected to said generator 'for changing the frequency of oscillation thereof, a reactance circuit for supplying quadrature voltages to the control electrode of said reactance tube from said oscillation generator, and a keying circuit connected to saidscreen grid for abruptly changing the voltage on said screen grid with respect to the cathode from one value when the keying circuit is open to another value when the keying circuit is closed.

8. In combination, an oscillationgenerator, a pair of reactance tubes-each having an anode, a

cathode, a control electrode and an auxiliarygrid; means subjecting the electrodes of said reactance tubes to suitable operating potentials, means for feeding quadrature voltages of opposite sign and derived from the generator to the control grids of said reactance tubes, and a keying circuit connected to said auxiliary grids for rendering one of said reactance tubes conductive while rendering the other non-conductive and vice versa.

9. In combination, a resonant electrical circuit,

frequency of operation of said generator, a reactance tube having an anode, a cathode and a control grid, means connecting the cathode of the reactance tube and the cathode of the generator directly together, a common cathode return resister for said cathodes, means for connecting the a reactance tube having an anode, a cathode and a control electrode connected thereto for changing the eflfective resonant frequency of said circuit, a reactance circuit for supplying quadrature voltages to the control electrode of said reactance tube from said resonant circuit, a source of unidirectional potential having one terminal connected to the cathode of said reactance tube, a

resistance connecting another terminal of said source to another electrode of said reactancetube, and a key for eflectively connecting, at will, the electrode end of said resistance to stillanother terminal on said source of'potential. l

10. In combination, a resonant electrical cirior feeding quadrature voltages of opposite sign derived from vsaid circuit to the control grids of said reactance tubes, and a keying circuit including a plurality of resistors and a single source of potential for rendering at will one of said reactance tubes conductive while rendering the other non-conductive and vice versa.

11 In combination, an oscillation generator comprising an electron discharge system having an anode, a cathode and a grid, a lrequencyconcapacity to and trolling system having inductance plate of the reactance tube to the anode of said generator, means for subjecting the control grid of the reactance tube to a quadrature voltage with respect to the voltage on the plate of said reactance tube, and means for varying the gain of said.

reactance tube.

12. In combination, an oscillation generator having an electron discharge system including an anode, a cathode and a control grid, a circuit having inductance and capacity connected to said electrodes, means for subjecting said electrodes to suitable operating potentials whereby oscillations are regenerativly set up in said circuit having inductance and capacity, a reactance tube comprising an anode, a cathode and a grid, means connecting the anode of said reactance tube to said circuit having inductance and capacity,

means for deriving from and subjecting the control grid of said reactance tube-'toa quadrature voltage with respect to thevoltage at the point in said circuit having inductance and capacity to which the plate of said reactance tube is connected, a source of potential having an intermediatepoint connected to the cathode of said reactance tube, a resistor connecting another point on said source of potential to a grid electrode within said reactance tube, and a key for connecting the grid end of said resistor at will to still another point on said source of potential.

V cult, a pair of reactance tubes each having an anode, a cathode and a control electrode, means 13. In combination, an oscillation generator comprising a tube having an anode, a cathode and a grid, a circuit having inductance and capacity connected to the electrodes of said tube, means for subjecting the electrodes of said tube to suitable operating potentials whereby oscillations are regeneratively set up in said circuit having inductance and capacity, a reactance tube-having an anode, a cathode, a grid, and an auxiliary grid, connection from the anode of saidreactance tube to said circuit having inductance and capacity, a circuit connected to the anode and control grid of said reactance tube for subjecting the control grid to a quadrature potential with respect to the potential on the anode of said reactance tube, a

source of potential having an intermediate point connected to said cathode of said reactance tube,

a resistor connecting another point on said source -Y rumors connected to said electrodes for determining the 

